This invention relates generally to dynamo-electric machines and, more particularly, to processes for designing rotors of such machines.
A dynamoelectric machine, such as an AC induction motor, typically includes a rotor core which, in one known form, has opposed substantially planar end surfaces and a substantially cylindrical, longitudinally extending body portion. The rotor core also has a rotor shaft bore and a plurality of rotor bar slots. The rotor bar slots sometimes are referred to as secondary conductor slots.
The above described rotor core typically is formed by a plurality of steel laminations. More specifically, each lamination is stamped from a steel sheet, and has a central opening and a plurality of spaced, radially arranged openings adjacent the lamination outer periphery. The laminations are arranged in a stack so that the openings at the outer periphery of the laminations are aligned to form rotor bar slots and the central openings are aligned to form the rotor shaft bore. The rotor core can alternatively be formed of solid magnetic steel.
To complete the rotor formation process for a standard cast aluminum type rotor, rotor bars are cast in the rotor bar slots and end rings are cast at the opposing ends of the core using, for example, an aluminum casting process. The rotor bars typically extend through the slots and the end rings "short" the bars together at the ends of the rotor core. A rotor shaft extends into the rotor shaft bore and is secured to the rotor core by any suitable process, such as, for example, interference fit or keying. Such a rotor sometimes is referred to in the art as a "squirrel cage" type rotor.
In an X-ray tube, electrons are produced at a cathode by heating a filament. The electrons are attracted to an anode target by a high voltage potential difference (typically about forty to one hundred fifty kilovolts). When the accelerated electrons hit the anode target, X-rays are produced. Only about one percent of the electron energy is converted into X-ray radiation. The remaining energy is converted into heat. To avoid exceeding the melting point of the focal spot on the target where electrons hit, the target is rotated. An induction motor with a squirrel cage rotor can be utilized for rotating the X-ray tube anode. In X-ray tube motors, the rotor bars are usually made of copper due to high operating temperature of the rotor (250-450 .degree. C.) where aluminum resistivity would be too high. These copper bars are either cast (in a similar way to aluminum bars) or pre-fabricated.
In operation, an induction motor, including a squirrel cage type rotor, rotates at a target speed. A supply source impresses an alternating voltage on stator windings to create an alternating current in the stator windings. The alternating current generates an alternating magnetic field which induces currents in the rotor bars of the rotor. Current flow through the rotor bars results in the generation of magnetic fields. As is well known, the magnetic fields generated by the stator windings and the rotor bars couple and create a torque which causes the rotor to rotate. The stator and rotor operate as a rotating transformer with a secondary (rotor) whose secondary impedance is determined by the cross-sectional area of the rotor bars. The magnitude of the current in the stator windings is affected by the rotor impedance.
In the above described induction motor, the rotor rotates at a speed less than synchronous speed when the load torque is greater than zero. For example, in a six pole induction motor, the synchronous speed (for sixty hertz operation) is 1200 rpm. The rotor may, however, have an actual steady state speed of 1100 rpm. Such a condition is known as "slip." Factors affecting steady state slip of the rotor include the stator voltage (current), bearing friction, and rotor unbalance. For X-ray tube applications, during transients such as anode acceleration and braking, the stator voltage (current) and frequency profile together with the anode assembly rotational moment of inertia and rotor temperatures have the greatest influence on the rotor slip.
In an X-ray tube system, the rotational speed of the anode preferably is precisely controlled so that an operator can prevent the anode from overheating. If the anode overheats, a scan may have to be interrupted to allow the anode to cool. Interrupting a scan, of course, is highly undesirable. Overheating of the anode can also result in degradation of image quality.
In addition, it is also highly desirable for the motor performance to be optimized so that the anode reaches its peak speed at a minimum possible time, thus enabling operation of the X-ray tube system at full power as quickly as possible with a high patient turn-over rate.
Variations in motor performance can result from variation in operating conditions (e.g., temperature, load torque), and variations in motor dimensions (e.g., due to variation in labor, machining tools). Variations in operating conditions are sometimes referred to as noise parameters. Variations in motor dimensions are sometimes referred to as manufacturing tolerances.
Williamson et al., "Optimization of the Geometry of Closed Rotor Slots for Cage Induction Motors," IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, Vol. 32, No. 3, May/June 1996, pp. 560-568, describes an optimization procedure to determine the design of a rotor bar slot to obtain maximum operating efficiency but does not appear to address manufacturing tolerances.
Moses et al., "A Computer-Based Design Assistant for Induction Motors," IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, Vol. 30, No. 6, November/December 1994, describes a computer-based design assistant for motor cost optimization using Monte-Carlo analysis but again does not appear to address manufacturing tolerances or capabilities.
Tucci et al., "A Simulator of the Manufacturing of Induction Motors," IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, Vol. 30, No. 3, May/June 1994, describes a simulator of the manufacturing process of induction motors which includes providing feedback in the form of motor cost and production scheduling. The description of the process appears to relate more to optimizing the factory itself rather than optimizing the design of a motor.